Error in Implementing singleton pattern

By declaring the __construct() function as private like private function __construct() you are effectively disallowing PHP from automatically calling it upon object creation.

Instead, you should always have your __construct() as well as other magic methods declared public.

public function __construct() { // Now PHP can access me } 

As far as making your Database class follow a singleton pattern, it doesn't make sense to extend a class that doesn't (i.e. PDO). Instead, do something like this:

<?php class Database { private static $instance; public function __construct() { // Check to see if static PDO instance // has already been created, so we create only one (singleton) if(!self::$instance) { self::$instance = new PDO( "mysql:host=localhost;dbname=live", "root", "root" ); } } public function getInstance() { if(self::$instance === null) { self::__construct(); } return self::$instance; } public function __clone() { die(__CLASS__ . ' class cant be instantiated. Please use the method called getInstance.'); } } $mySingleton = Database::getInstance(); var_dump($mySingleton); ?> 

As PDO's __construct() function is public, you cannot extend it with a private __construct() function.

So a "real" singleton is not possible.
You have to set public function __construct().

The need to have different access level between parent and child constructors is perfectly lecit, apart from technical difficulties involving the PHP source code.

In fact, it has been filed as a bug (#61970).

In this commit made on march 2017, if I am not wrong, the developer says it has been closed:
http://git.php.net/?p=php-src.git;a=commitdiff;h=5324fb1f348f5bc979d9b5f13ac74177b73f9bf7

I use PHP 7.0.24 released on july, but I still see it has not been fixed.

You should

1) disable constructor by setting it to private.

2) create single PDO object by calling the static method only. Static method have to return the instance of PDO object.

In Silex or Symfony you will have to prefix class name with "\" or use "use \PDO;". Which means nothing more that this is a global class.

Ps. if you set the __constructor to public and use return functino note that it will not generate any exception or warning but you will get an class object returned and not actual value of return statement.

So $db = new Database() would return object of class Database. Then from there you would have to access your PDO using class method. $pdo = $db->getInstance() Which is not the right way to build proper singleton.

If you interested in reading more about singletons pros and cons and some use cases read this Best practice on PHP singleton classes, you will find more information about this pattern design.

/** * Singleton pattern */ class Database { /** * holds the only PDO instance */ private static $_instance; /** * private __constructor not accesible */ private function __construct() { self::$instance = new PDO( "mysql:host=localhost;dbname=live", "root", "root" ); } /** * clone will not duplicate object */ public function __clone() { die(__CLASS__ . ' class cant be instantiated. Please use the method called getInstance.'); } /** * the only public function to access or create PDO object */ public static function getInstance() { if(!self::$_instance instanceof PDO){ new self; } return self::$_instance; } }